1. Technical Field
The present invention relates to an electric-current-control type variable inductor which is used in electronic tuning circuits such as car radio tuners.
2. Background Arts and its Problems
FIG. 3 is a view used to explain a prior art electric-current-control type variable inductor which has been applied by the same applicant (Japanese Patent Application No. 59-14822).
A first core 1 is mounted with a control coil 2 and a second core 5 mounted with a tuning coil 4 is inserted into a hollow portion 3 defined by a winding portion. The first core 1 is covered with a pot-shaped third core 6. The first and second cores 1 and 5 are mounted on a base 7 and the third core 6 is fitted over the base 7. Terminal pins 8 are extended downwardly from the base 7. The magnetic path established by the control coil 2 is extended through the first and third cores 1 and 6 as indicated by the broken lines 9 while the magnetic path established by the tuning coil 4 is extended through the second core 5 and the first core 1 as indicated by the broken lines 10. Both the magnetic paths 9 and 10 are most superposed one upon another at the winding portion 11 of the second core 5. When a DC current or a low-frequency current is made to pass through the control coil 2 to vary a magnetic flux density, the effective permeability of the second core 5 is controlled, whereby the inductance of the tuning coil 4 can be varied. The variable ratio of inductance; that is, a value obtained by dividing a maximum value by a minimum value is increased when a low-frequency core having a high magnetic flux density is used as the first core. On the other hand, a high-frequency current passes through the tuning coil 4 so that a high-frequency core having a low magnetic flux density must be used as the second core 5 so as to minimize losses.
However, in the case of the variable inductor of the type described above with reference to FIG. 3, the magnetic path 10 established by the tuning coil 4 is extended into the first core so that the loss at the portion of the first low-frequency core at which the magnetic path 10 is established is increased. In addition, the increase in inductance and Q factor and a variable ratio of the tuning coil 4 is limited.